1. Field of the Invention
The present invention relates to the surface treatment of plastic articles, and more particularly, concerns a method and apparatus for treating a surface of a plastic article so that the surface has enhanced printing and adhesion characteristics. The present invention also relates to the articles made in accordance with the method of the invention.
2. Description of the Prior Art
In the use of plastic materials for packaging, containers, tubular structures, medical products and a variety of other products and uses, it is desirable to print on the surface of these plastics. The most common approach for printing on the surface of plastics, such as polyethylene, involves printing inks. These inks, however, have not satisfactorily adhered to the normal surface of the plastic articles onto which printing is desired. As a result, not only was the quality of the printing poor, but the ink could be readily rubbed off the surface of the article.
Investigators about thirty years ago determined that surface treatment of the plastic would enhance printability and adhesion of the printing inks. In particular, a corona discharge treatment of the surface of polyethylene films was found to improve the compatibility of the surface with printing inks. To achieve such surface treatment by the corona discharge approach, the plastic material is introduced into a region of ionized air in a gap between two charged electrodes. High-energy particles bombard the surface of the plastic material in the ionized region. Covalent bonds on the surface of the material are broken as a result of the particle bombardment energies developed in the ionized region. Free radicals that are formed on the surface rearrange to form functional groups which directly affect surface wetting and adhesion. Molecular transformations and dissociations may form species with a chemically altered surface favorable for adhesion. Increased adhesion is the result of an electrical formation induced in the polymeric material by the corona discharge. In simplified terms, the particle bombardment on the surface of the plastic material creates microfissures, increased porosity and polar groups which improve adhesion and surface wettability characteristics.
Although the knowledge and techniques of corona discharge treatment of plastic materials have been in existence for approximately thirty years, there is no known technique, process or equipment for the treatment of individual or formed plastic articles that are in continuous motion, preferably at high speeds which would be desirable for many commercial automated operations. It has been known, for example, to treat the surfaces of plastic films, such as polyethylene, which are moving through the corona discharge region. British Pat. No. 765,545 discloses such a process in which the surface of a polyethylene or other plastic film is treated to render it receptive to receive printing ink, coloring, adhesive or the like. In U.S. Pat. No. 4,392,178, an apparatus is described for rapidly enhancing the piezoelectric properties of polar polymeric films by continuous corona poling thereof. In the patented invention a field-intensified ionization (corona) source of voltage repeatedly traverses the moving polarizable polymer to result in fewer breakdowns of the polymer film when compared to prior art static corona apparatuses.
Various types of research or testing have been performed by investigators to correlate the effects of corona treatment and the increase in adhesion characteristics. For instance, Baum et al. reported on research which they performed on the decay of surface potential of negative corona charged polyethylene films, "Further Observations on the Decay of Surface Potential of Corona Charged Polyethylene Films," J. Phys. D.: Appl. Phys., vol. 10, 1977, pp 2525-2531. Baum et al. employed an airstream directed along the surface of the polyethylene film during the corona discharging. Polyethylene films were also corona discharge treated by Carley et al. in order to study the adhesion phenomena produced by the corona treatment, "Corona-Discharge Treatment of Polyethylene Films. 1. Experimental Work and Physical Effects," Polymer Engineering and Science, March 1978, vol. 18, number 4, pp 326-334. Stradal et al. also conducted adhesion experiments on low density polyethylene sheet for evaluating the effect of corona treatment in oxygen, nitrogen, helium and argon on ink adhesion to the polyethylene sheet, "The Effect of Corona and Ozone Treatment on the Adhesion of Ink to the Surface of Polyethylene," Polymer Engineering and Science, January 1977, vol. 17, no. 1, pp 38-41. Ehrbar et al. studied the application of corona discharge on the metalization of plastic materials, in order to promote good adherence between the plastic material and the metallic coating, "Using Corona Discharge to Deglaze Plastic Films Before Metalizing," Plating and Surface Finishing, February 1980, pp 64-66.
Even though the surface treatment of moving plastic films has been known in order to enhance printing and adhesion characteristics, there are no known techniques which treat moving articles such as containers, packages, tubes or other structural bodies which are formed independently of each other and are moving through the treatment zone. When continuous plastic films are treated to improve printability or adhesion, the effect of the corona discharge process is to modify one side or surface of the film so that the printing ink on th modified surface will print and adhere in improved fashion. On the other hand, containers, packages, tubes and the like have many sides, or one continuous side or surface if the the container is round or cylindrical, onto which printing is desirable. Therefore, since finished or formed articles made out of plastic such as polyethylene, polypropylene and the like, need to be treated around the entire peripheral surface (360.degree. coverage), the known techniques of treating flat films are inapplicable in many respects.
For instance, Baum et al., mentioned above, found that when air was blown across the surface of a film sample they were able to increase the charged area by 50%. They also reported that when charging is performed in moving air, excited molecules will be dispersed and their influence will be greatly reduced. Therefore, they reasoned that the time requirement to produce sufficient charging will be increased due to the dispersion of the excited molecules. Similarly, when charging is performed on a moving substrate, as opposed to moving air, the number of excited molecules striking the surface is dramatically reduced. The photons that are generated in a corona discharge are not deflected by a current of air. As a result, photon bombardment tends to create high and low intensity treatment areas between the electrodes.
Such high and low intensity treatment areas are, however, magnified with the introduction of a nonuniform, geometrically configured substrate or article. Difficulty thus results in treating the surface of a geometrically configured article that is positioned next to the electrode without rotation of the article itself, if the presently known corona treatment processes are employed. Furthermore, it is known that present corona treatment processes employ a very small airflow to help initiate the corona field. This airflow is typically directed transversely across the substrate. This airflow tends to deflect the excited gas molecules away from the substrate so that photons are relied upon solely for surface treatment. This results in a nonuniformly treated surface which is unacceptable for treating articles which need 360.degree. coverage for improved adhesion characteristics.
Other problem areas also arise in the employment of corona discharge treatment apparatuses. Heat generated by the electrodes during operation must be controlled, otherwise the energy dissipated by the discharge may quickly overheat the substrate and cause premature failure of the dielectric electrode covering. This, in turn, creates a nonuniform corona field with hot spots which ultimately causes an electric arc. Also, ozone is a bi-product of corona discharge. Insofar as the production of ozone is an environmental hazard and regulated by law, ozone containment is a requirement for corona discharge treatment equipment.
It can be seen improvements are still needed and being sought to improve surfaces of plastic materials for the enhancement of printing and adhesion. Such improvements would be most desirable to treat the surfaces of formed plastic articles, as individual components, particularly at speeds suitable for automated commercial production capabilities. It is to such improvements, processes and articles that the present invention is directed.